Moisture testing before a concrete coating

A coating that blisters in month fourteen almost never failed because of the coating. It failed because water vapour moved up through the slab and pushed it off, and the warranty claim gets refused because nobody can show what the slab was doing on the day.

What the manufacturers actually require

Most warranties exclude damage from moisture vapour emission above roughly 7 to 8 pounds per 1,000 square feet per 24 hours, and most require that you tested. Not that you looked at it. That you tested, and kept the result.

Two methods are accepted in practice:

• Calcium chloride (ASTM F1869) gives you pounds per 1,000 per 24

hours. Three tests for the first 1,000 square feet, one more per additional 1,000.

• In-situ relative humidity probes (ASTM F2170) measure inside the

slab and are the better answer on anything thick or on grade.

Keep the evidence, not the memory

Photograph the test in place with the slab visible. Write down the reading, the date, the ambient temperature and the humidity. If the job goes wrong two years later, that photograph is the difference between the manufacturer covering a redo and you covering it.

What to do when the number is high

Do not coat and hope. The options, cheapest first:

• wait, if the slab is new and the building is closed in
• a moisture-tolerant primer or vapour-reduction coating
• a full vapour barrier system, which changes the price of the job

All three are quotable. Coating over a wet slab is not.

Prep is the other eighty per cent

Of the failures that are not moisture, most are profile. Grind to the profile the product sheet asks for — the ICRI CSP scale is what the warranty document means when it says "properly prepared" — and vacuum rather than sweep. The cheapest line to cut on a quote is prep, and it is the one that comes back.